Chair back



FIG. 1 is a back perspective view of a chair embodying the design;

FIG. 2 is a left side view of the chair of FIG. 1, with the right sidebeing a mirror image;

FIG. 3 is a front view of the chair of FIG. 1;

FIG. 4 is a back view of the chair of FIG. 1; and,

FIG. 5 is a top view of the chair of FIG. 1.

The non-design portions of the article are shown with broken lines.

The ornamental design for a chair back, as shown and described.